Semiconductor memory devices are a vital microelectronic component commonly found in digital logic systems, such as computers, and microprocessor-based applications ranging from satellites to consumer electronics. Therefore, advances in the fabrication of semiconductor memory devices, including process enhancements and circuit-design-related developments that allow scaling to higher memory densities and faster operating speeds, help establish performance standards for other digital logic families.
Semiconductor memory devices generally include volatile memory devices, such as random access memory (RAM) devices, and nonvolatile memory devices. In RAM devices, data is stored by either establishing the logic state of a bistable flip-flop such as in a static random access memory (SRAM), or by charging a capacitor in a dynamic random access memory (DRAM). In both SRAM and DRAM devices, data remains stored and may be read as long as the power is applied, but data is lost when the power is turned off.
Mask read-only memory (MROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), and electrically erasable programmable read-only memory (EEPROM) devices are capable of storing data even with the power turned off. The non-volatile memory data storage state may be permanent or reprogrammable, depending upon the fabrication technology used. Non-volatile semiconductor memories are used to store program and microcode storage in a wide variety of applications in the computer, avionics, telecommunications, and consumer electronics industries. A combination of single-chip volatile as well as non-volatile memory storage modes is also available in devices such as non-volatile SRAM (nvRAM) for use in systems that require fast, reprogrammable non-volatile memory. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance for application-specific tasks.
Mask read-only memory (MROM), programmable read-only memory (PROM) and erasable programmable read-only memory (EPROM) are not free to erase and rewrite so it is not easy to update the contents of the memory. On the other hand, electrically erasable programmable read-only memory (EEPROM) devices are electrically erasable and writable and may be readily applied to auxiliary memories or system programming memories that require continuous update.